The present invention relates to a joint assembly for joining a ceramic membrane of tubular form to a tube sheet used in supporting the ceramic membrane within a reactor. More particularly, the present invention relates to such a joint assembly in which the ceramic membrane is received and sealed within a fixture connected to the tube sheet.
Ceramic membranes are used to separate a gaseous component from a mixture, for instance, oxygen from air. Ceramic membranes are gas tight and function by allowing ions to selectively migrate through the membrane. The disassociation and ionization of the selected gas occurs at a membrane surface where electrons are picked up from near surface electronic states. The flux of the ions is charged compensated by a simultaneous flux of electronic charge carriers through the ceramic membrane. When the ions arrive at the opposite side of the membrane, the individual ions release their electrons and recombine to form gas molecules. The driving force for such transport can be a differential partial pressure of the selected gas applied across the membrane or an external source of electric power.
Ceramic membranes can be employed in the form of tubes located within reactors. Critical to the success of the reactor is both the survival of such ceramic membranes and adequate sealing at a location where the tubular ceramic element is joined with the reactor, generally at a tube sheet. Since ceramic membranes exhibit ion conductivity at temperatures that are well above 500xc2x0 C., generally in the range of between about 600xc2x0 C. and about 1000xc2x0 C. the joint assembly and seal between the ceramic membrane and the tube sheet are subjected to extreme environmental conditions.
A major obstacle in developing a viable seal are the unique thermomechanical properties of ceramic materials, e.g. high thermal expansion and compositional dilation and the high operational temperatures of the membranes. Both factors prohibit the use of common fixed joining techniques such as glass sealing or brazing. Instead, joining techniques that do not rigidly affix the ceramic within the reactor are used such as non-bonding, compression type joint assembles. For instance, both U.S. Pat. Nos. 5,820,654 and 5,820,655 use either a sliding or fixed seal with a bellows at the juncture of the ceramic membrane and the tube sheet.
The prior art has provided high temperature seals that are used to seal structural panels. For instance, U.S. Pat. No. 4,917,302 utilizes a stack of ceramic wafers located within a rectangular groove along the side of a movable engine panel. The engine panel is sealed to an adjacent side wall by the ceramic wafers which are held in position by a pressurized linear bellows that also fits within the groove. U.S. Pat. No. 5,082,293 shows a similar seal except that the sealing element instead of consisting of a stack of wafers is made up of multiple layers of a fiber wound about a core. The materials for such fibers can be alumina-boriasilicate or silicon-carbide. U.S. Pat. No. 5,301,595 discloses a rope seal having a core of ceramic fibers and a cover of stainless steel. The rope seal is designed to seat within a groove in one component and bear against a flat wall of another component. U.S. Pat. No. 4,394,023 shows a high temperature valve stem packing that incorporates graphite seal rings composed of coiled graphite tape held between metal packing adapter rings that bear against the graphite seal rings.
All of the foregoing materials are attractive for sealing a ceramic membrane to a tube sheet due to their high temperature performance. As will be discussed, the present invention provides a joint assembly that is designed to advantageously utilize high temperature sealing materials, such as those identified above and to hold the ceramic membrane in place.
The present invention provides a joint assembly for joining a ceramic membrane of tubular configuration to a tube sheet. The joint assembly has a fixture connected to the tube sheet and including a passageway having a narrow end section, located at one end of the passageway, to receive the ceramic membrane. A sealing surface, defined by the passageway, is located at the one end thereof. The ceramic membrane extends through the narrow end section of the passageway so that an open end of the ceramic membrane is located within the passageway and an adjacent lateral surface of the membrane is surrounded by the sealing surface. At least one sealing element is located between the sealing surface and the lateral surface of the ceramic membrane. A follower is located within the passageway and bears against the at least one sealing element in a direction towards the narrow end section of the passageway. This action simultaneously drives the at least one sealing element against the sealing surface and the lateral surface of the ceramic membrane by compression of the at least one sealing element. As a result, a seal is effected between the fixture and the ceramic membrane and the ceramic membrane is held in place by frictional forces developed between the at least one sealing element and the ceramic membrane. The follower is provided with an inner passage in communication with the open end of the ceramic membrane to allow permeate or feed to flow through the follower.
The sealing surface can be formed by a tapered section of said passageway, tapering towards the narrow end section. In such case, the passageway is also provided with an annular end surface connecting the tapered section of said passageway with the narrow end section. The follower has a fusto-conical end element configured to fit within said tapered section of said passageway and to bear against said at least one sealing element.
In a further aspect of the present invention the fixture bears against the ceramic membrane to compress the at least one sealing element. In this aspect of the present invention, the fixture of the joint assembly includes a passageway having a narrow end section, located at one end of the passageway. A sealing surface surrounds and is located adjacent to the narrow end section. The ceramic membrane has an enlarged end portion at an open end thereof. The enlarged end portion is located within the passageway with the ceramic membrane extending from the narrow end section of the passageway. At least one sealing element is located between the sealing surface and the enlarged end portion of the ceramic membrane and a follower, located within the passageway, bears against the enlarged end portion of the ceramic membrane in a direction towards the narrow end section of the passageway. This action compresses the at least one sealing element between the sealing surface and the enlarged end portion of the ceramic membrane. As a result, a seal is effected between the fixture and the ceramic membrane and the ceramic membrane is held in place, against the sealing surface. The follower is provided with an inner passage in communication with the open end of the ceramic membrane to allow feed or permeate to flow through the follower.
The enlarged end portion of the ceramic membrane can be formed by an outwardly flared portion of the ceramic membrane to produce an outwardly flared lateral surface thereof. In such embodiment of the invention, the sealing surface is formed by a tapered section of the passageway, tapering towards the narrow end section. The at least one sealing element is a cone seal gasket located between said outwardly flared lateral surface and said sealing surface. An annular butt seal gasket is located between the follower and the open end of said ceramic membrane in alignment with the inner passageway of the follower.
In yet another alternative embodiment the sealing surface is an annular end surface of the passageway surrounding the narrow end section thereof. The enlarged end portion has an annular undersurface located opposite to the annular end surface and the at least one sealing element comprises a annular butt seal gasket located between said annular undersurface and said annular end surface. The follower also has an end section having a cavity configured to receive the enlarged end portion of said ceramic membrane. In such embodiment, the enlarged end portion of the ceramic membrane and the cavity can be of fusto-connical configuration. In such case, a cone-seal gasket is located between the enlarged end portion and the cavity. A butt seal gasket is located between the open end of the ceramic membrane and the follower, in alignment with the passage thereof.
The at least one sealing element can be formed of a rope-like packing wound around the lateral surface of the ceramic membrane. Alternatively, the at least one sealing element can be formed of a paper or felt stuffing of the ceramic material. The ceramic material can be an aluminosilicate fiber or a zirconia fiber. Advantageously, the ceramic material can be infiltrated with a particulate material and preferably such particulate material can be a ceramic or a metal. The sealing element can generally be formed of a ceramic mineral such as vermiculite. The at least one sealing element can also be formed of a layer of a ceramic powder or a graphite packing.
In any embodiment of the present invention, the fixture can be provided with an inlet port for introduction of a buffer gas into the fixture.